The broad, long-term objective of the proposed research is to determine the structural and mechanistic basis for antigen presentation by class II MHC proteins. The proposed research involves in vitro biochemical and biophysical analysis of recombinant class II MHC proteins, associated chaperonins, and peptide exchange factors. The specific research proposed is intended to characterize peptide-induced conformational changes in HLA-DR1 as it binds peptides and as it converts between peptide-receptive and inactive forms, to determine how DM catalyzes peptide exchange on class II MHC proteins, and to evaluate possible non-structural interactions in the MHC peptide-binding site. These aims will be pursued through spectroscopic and hydrodynamic analysis of intermediates in the peptide binding reaction, kinetic studies of catalyzed and uncatalyzed peptide binding and release reactions, and structure determinations for class II MHC proteins in complex with peptides and associated proteins. Detailed knowledge of these interactions would improve our ability to predict peptide binding preferences of MHC proteins and thus the potential antigenicity of protein therapeutics and autoantigens, and would aid efforts to design molecules that promote or interfere with peptide binding and antigen presentation in vivo.